GOODWILL AND OTHER INTANGIBLE ASSETS

The following table summarizes the carrying amount of goodwill by segment:



(in millions)	Environmental Solutions			Safety & Security Systems			Total
Balance at December 31, 2015	$	120.4		$	111.2		$	231.6
Acquisitions	7.1			—			7.1
Translation adjustments	(0.3		)	(1.9		)	(2.2		)
Balance at December 31, 2016	127.2			109.3			236.5
Acquisitions	136.0			—			136.0
Translation adjustments	0.4			4.4			4.8
Balance at December 31, 2017	$	263.6		$	113.7		$	377.3

The following table summarizes the gross carrying amount and accumulated amortization of intangible assets for each major class of intangible assets:



	December 31, 2017							December 31, 2016
(in millions)	Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Definite-lived intangible assets:
Customer relationships^(a)	$	90.9	$	(4.6	)	$	86.3	$	0.8	$	(0.1	)	$	0.7
Other^(a)	2.9		(0.9		)	2.0		1.1		(0.4		)	0.7
Total definite-lived intangible assets	93.8		(5.5		)	88.3		1.9		(0.5		)	1.4
Indefinite-lived intangible assets:
Trade names	63.5		—			63.5		8.8		—			8.8
Total indefinite-lived intangible assets	63.5		—			63.5		8.8		—			8.8
Total intangible assets ^(b)	$	157.3	$	(5.5	)	$	151.8	$	10.7	$	(0.5	)	$	10.2


(a)	Average useful life of customer relationships and other definite-lived intangible assets are estimated to be approximately 12 years and 6 years, respectively. The average useful life across all definite-lived intangible assets is estimated to be approximately 12 years.


(b)	Amounts at December 31, 2017 include intangible assets acquired in the TBEI acquisition - see Note 2 – Acquisitions.

Amortization expense for the year ended December 31, 2017 was $4.8 million. Amortization expense for the years ended December 31, 2016 and 2015 was immaterial.

The Company currently estimates that aggregate amortization expense will be approximately $8.0 million in 2018, $8.0 million in 2019, $8.0 million in 2020, $8.0 million in 2021, $7.7 million in 2022 and $48.6 million thereafter. Actual amounts of amortization may differ from estimated amounts due to additional intangible asset acquisitions, changes in foreign currency rates, impairment of intangible assets and other events.